Hammer Milling and Jet Milling Fundamentals

Select the optimal hammer mill or jet mill for your application by characterizing the feed material and conducting milling tests.

Size reduction, or comminution, by mechanical crushing and grinding is an important unit operation in the chemical, power, mineral, metallurgical, and pharmaceutical industries. Throughput for an individual device can range in scale from a few kilograms per hour for specialty products to hundreds of tons per hour for ore-extraction operations. It is estimated that size reduction accounts for approximately 5% of global energy consumption.

Many sources (1–4) describe the fundamentals of material size reduction, but few teach you how to select and size the right type of mill, how to operate your mill efficiently, and how to maintain your mill.

There are numerous types of grinding mills. This article focuses on two widely used types: the hammer mill and the jet mill. Hammer mills are often used for general-purpose grinding and the finished product particle size ranges from millimeters to tens of microns. The jet mill is primarily used for superfine grinding applications and creates particles sizes down to a few microns. Understanding the fundamentals of each mill’s respective grinding mechanisms, operating philosophies, and best maintenance practices is essential to achieving good product quality, energy savings, and high throughput.

Several pieces of information are needed to select and size a mill. You must know the required annual capacity, whether batch or continuous operation will be required, which upstream and downstream processes will be needed for the mill, and the properties (such as size and shape) of the finished product. Fully characterizing the feed material is essential to identifying the right mill for the job.

This article gives a detailed description of hammer mill and jet mill operation, and describes how to characterize the feed material and control product particle size.

Understand the size-reduction application

Before designing a milling system, or making any purchases, you must fully understand the requirements of your size-reduction application. Ask yourself:

• Why do I need to perform size reduction?
• Can I buy the right-sized material cheaply and directly from any suppliers?
• Can the superfine particle be made more inexpensively from a bottom-up method, such as synthesis?

If the answers to these questions indicate a size reduction step in your process is essential, you should verify the other process requirements, such as production capacity, product particle size, and particle-size distribution.

The capacity requirement of the milled product in your process must be illustrated clearly in a yearly, monthly, daily, and hourly rate. The capacity not only determines the mill equipment size, but also dictates how to operate the mill, whether continuous or batch operation is required, and whether the process can be performed in-house or if a tolling service is a better option.

The required product size is an important grinding cost factor, especially for fine-size grinding. You should evaluate particles in several size ranges to determine the effect of size on product quality. In general, the grinding cost increases significantly as the product particle size decreases. The coarsest size conducive to good product quality should be used whenever possible.

Sometimes, particle shape, bulk density, or flowability are also important to the quality of the final ground product. Bulk density and flowability are strongly associated with particle shape. In general, smooth, round particles yield high bulk density and good flowability. The particle shape is determined by the properties of the feed material and the size-reduction mechanism of the mill. However, because the feed material typically cannot be modified, particle shape is strongly influenced by the type of grinding mill selected.

Characterize the feed material

To determine the type of mill you will need for your application, gather information about the nature of the material to be ground. Certain properties are important, such as hardness, toughness, stickiness, melting point and thermal instability, explosibility, toxicity, particle size and particle-size distribution, flowability, and bulk density.

Hardness/softness. The Mohs scale of mineral hardness is frequently used to specify hardness. A hammer mill is typically good for grinding softer materials with Mohs hardness ranging from 1 to 5, while a jet mill can grind materials with Mohs hardness as high as 10. High hardness materials become very abrasive; therefore, they are not suitable for high-speed hammer mills. In a typical jet mill, grinding action is caused mainly by particle-particle collisions, so wear to the mill’s internals is less of an issue.

Toughness/brittleness. Toughness is a material’s ability to absorb energy and plastically deform without fracturing. A feed stream containing a material with a high toughness (such as rubber) may need to be cooled — which reduces molecular mobility and increases brittleness — before grinding the material. Liquid nitrogen is often used to cool the feed stream in hammer mills for grinding tough materials. However, liquid nitrogen is seldom used in jet milling operations due to the swift heat exchange between the material and the grinding gas. The grinding gas heats up the super-cooled feed stream, and the material loses its brittleness when it absorbs this heat.

Stickiness. This property is sometimes related to moisture content. However, stickiness can also increase with higher fineness. Material with high stickiness creates problems in all aspects of the grinding process, including feeding (e.g., making metering into the mill difficult), grinding (e. g., plugging the hammer mill screen or blocking the air classifier of a jet mill), and collection (e.g., plugging the bag filters).

There are two ways to grind sticky materials. The first solution is to dry the material prior to grinding, or dry and grind it at the same time by sweeping hot air through the mill. As an alternative, water can be added to the material and it can be ground wet; this solution is suitable for hammer mill operation but not for jet mills.

Melting point and thermal instability. Jet mills can grind materials with low melting points effectively because these mills have intrinsic cooling due to compressed air expansion in the body of the mill. With the help of sweeping air, hammer mills may also be able to process materials with low melting points for coarse grinding applications.

Explosibility. The risk of dust explosions must be taken into account when a material is ground very fine in a milling process. Dust-laden air can give rise to devastating dust explosions. Unless the material is one of the few commonly known to present no dust explosion risk (a stable oxide, for example, such as titanium dioxide or sand), you should always test the material’s explosibility characteristics.

The material’s dust deflagration index (K_st ) and maximum explosion pressure (P_max ) values should be used to design any necessary dust explosion protection devices, such as active suppression systems, explosion vents, and isolation systems. The minimum ignition energy (MIE) can provide guidance for selecting a safe unloading method for materials received in bags, bulk bags, or drums. Materials with a low MIE (e.g., <10 mJ) require special equipment and operational precautions for handling, packing, and unpacking the material. Targeted use of blanketing with an inert gas, such as nitrogen, is another strategy to help prevent combustible dust explosions. Electrical grounding and bonding on related equipment and pipes are also important to eliminate static charges, which are a source of ignition.

Dust explosion risks may be even more serious in plant rooms containing the milling machinery than in the machinery housing itself. The hazard arises from bad house-keeping, which allows dust to accumulate on ledges, ceiling beams, and other surfaces. After an initial dust explosion in or around milling equipment, a second explosion can occur from the dust that has been allowed to accumulate in the plant room. Although the first explosion may be minor, it disturbs the dust on surfaces throughout the room, which fills the room with a dust cloud. The dust cloud is then ignited by an ignition source, typically a small fire generated by the first explosion. The secondary explosion is usually far more devastating than the first explosion, because a much greater volume of dust is involved. Good housekeeping is the best way to prevent secondary dust explosions.

Toxicity. Understanding your material’s toxicity is critical to protecting both workers and the environment. For a highly toxic material, a totally enclosed milling system is recommended. Enclosed systems are available for both hammer mill and jet mill operations. A product collection bag filter...